Abstract: The 20th century witnessed several major advances in medicine. Perhaps most important were the discovery of antibiotics for bacterial infections and effective vaccines for several major viruses. Unfortunately, the creation of effective vaccines for bacteria has lagged behind analogous anti-viral strategies. Compounded with the rise in antibiotic resistance and a lack of interest from the pharmaceutical industry in pursuing novel antibiotics, we risk losing the fight against bacterial pathogens. Described herein is an unconventional strategy to exploit bacterial toxins as both novel targets for antibacterial agents and antigens for vaccine development. To intelligently address the increasing threat posed by bacterial pathogens, more effort is needed to uncover the molecular underpinnings of virulence. Our group specializes in the use of bioinformatics, in vitro reconstitution, and genetic manipulation to identify and characterize gene clusters that are responsible for the biosynthesis of virulence- promoting cytolysins. The best-known toxin in this family is the highly modified peptide, streptolysin S (SLS, produced by Streptococcus pyogenes). SLS production is required for the infective process, but not essential life processes. Our work has uncovered SLS-like toxins are synthesized by at least three other notorious human pathogens, including Staphylococcus aureus, Listeria monocytogenes, and Clostridium botulinum. We aim to study the potential role of the SLS-like toxin in an additional organism, Borrelia burgdorferi (Bb), which causes Lyme disease. Although widely known, the Bb molecular mechanism of pathogenesis is inadequately defined. If the SLS-like toxin was indeed employed during Bb infections, this would represent the first demonstration of toxin utilization in this family of organisms and would prompt a major revision of borrelioses. Because bacteria typically employ disparate pathogenic mechanisms, the conserved, SLS-like pathway provides a rare opportunity to develop more broadly applicable, yet targeted countermeasures. From our perspective, new antimicrobial strategies should directly target the pathogenic mechanism, rather than DNA replication, protein synthesis, or the cell wall. This approach holds enormous potential, as these drugs will theoretically be resistant to resistance. This project will identify inhibitors of SLS toxin biosynthesis for the specific purpose of developing novel antibacterials. Moreover, SLS is non-immunogenic, rendering it an unfeasible candidate for vaccine development. We have succeeded in generating attenuated variants with the anticipation that these can be used for raising toxin-neutralizing antibodies. The notion of immunizing against a bacterial toxin represents a potentially general strategy for future vaccine development. With this proposal, we aim to not only fundamentally shift the accepted view of Bb pathogenesis, but also to challenge the paradigm that antibiotics must kill bacteria and non-immunogenic toxins are intractable vaccine candidates. These seemingly unrelated goals are actually quite intertwined. Our approach rests on the philosophy that a more complete understanding of toxin biosynthetic pathways and chemical structure can be rationally exploited to design novel therapeutics. Public Health Relevance: Bacterial pathogens employ numerous mechanisms to evade the human immune system. We have discovered a novel strategy within the organism that causes Lyme Disease, who's pathogenesis remains largely enigmatic. A greater understanding of these processes will lay the foundation for developing the next generation of antimicrobial drugs.